Thermodynamic Analysis of Nickel(II) and Zinc(II) Adsorption to Biochar

May 10, 2018 - ACS eBooks; C&EN Global Enterprise ..... For this reason, the application of empirical partitioning constants to predict metal distribu...
1 downloads 0 Views 1MB Size
Subscriber access provided by the University of Exeter

Environmental Processes

Thermodynamic Analysis of Nickel (II) and Zinc (II) Adsorption to Biochar Md. Samrat Alam, Drew Gorman-Lewis, Ning Chen, Shannon L. Flynn, Yong Ok, Kurt Konhauser, and Daniel S Alessi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06261 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34

Environmental Science & Technology

1

Thermodynamic Analysis of Nickel (II) and Zinc (II) Adsorption to Biochar

2 3

Md. Samrat Alam,1 Drew Gorman-Lewis,2 Ning Chen,3 Shannon L. Flynn,1,4 Yong Sik Ok5, Kurt O. Konhauser,1 Daniel S. Alessi1*

4 5 6 7 8 9 10 11 12

1

Department of Earth & Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Alberta, T6G 2E3, Canada 2 Department of Earth and Space Sciences, University of Washington, Johnson Hall Rm-070, Box 351310, 4000 15th Avenue, NE Seattle, WA 98195, USA 3 Canadian Light Source Inc., University of Saskatchewan, 114 Science Plane, Saskatoon, SK, S7N 0X4, Canada 4 School of Natural and Environmental Sciences, Newcastle University, Newcastle upon Tyne, NE1 7RU, United Kingdom 5 Korea Biochar Research Center, OJeong Eco-Resilience Institute & Division of Environmental Science and Ecological Engineering, Korea University, Seoul 02841, Korea

13 14 15 16 17 18

19

20 21 22 23 24 25 26 27 28

ACS Paragon Plus Environment

1

Environmental Science & Technology

29

ABSTRACT

30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

While numerous studies have investigated metal uptake from solution by biochar, few of these have developed a mechanistic understanding of the adsorption reactions that occur at the biochar surface. In this study, we explore a combined modeling and spectroscopic approaches for the first time to describe the molecular level adsorption of Ni(II) and Zn(II) to biochar using five types of biochar. Following thorough characterization, potentiometric titrations were carried out to measure the proton (H+) reactivity of each biochar, and the data was used to develop protonation models. Surface complexation modeling (SCM) supported by synchrotron-based extended X-ray absorption fine structure (EXAFS) was then used to gain insights into the molecular scale metal-biochar surface reactions. The SCM approach was combined with isothermal titration calorimetry (ITC) data to determine the thermodynamic driving forces of metal adsorption. Our results show that the reactivity of biochar towards Ni(II) and Zn(II) directly relates to the site densities of biochar. EXAFS along with FT-IR analyses, suggest that Ni(II) and Zn(II) adsorption occurred primarily through proton-active carboxyl (–COOH) and hydroxyl (–OH) functional groups on the biochar surface. SCM-ITC analyses revealed that the enthalpies of protonation are exothermic and Ni(II) and Zn(II) complexes with biochar surface are slightly exothermic to slightly endothermic. The results obtained from these combined approaches contribute to the better understanding of molecular scale metal adsorption onto the biochar surface, and will facilitate the further development of thermodynamics-based, predictive approaches to biochar removal of metals from contaminated water.

50 51 52 53 54 55 56 57 58 59 60

ACS Paragon Plus Environment

2

Page 2 of 34

Page 3 of 34

Environmental Science & Technology

61

1. INTRODUCTION

62

Biochar is a carbon-rich solid produced through the carbonization and/or pyrolysis of biomass

63

derived from a variety of feedstocks, including straw, wood, and organic industrial wastes.1-3 It

64

has proven effective in the removal of organic and metal contaminants from water,4-8 is

65

considered a prospective alternative to activated carbon (AC) for water treatment and soil

66

amendment purposes due to its lower production cost, and it is also thought to act as a global

67

“carbon-sink”.8-10 The efficiency of contaminant removal from water by biochar depends on

68

several factors, including contaminant concentration and the distribution and types of surface

69

functional groups, the latter of which can vary widely depending on the pyrolysis temperature

70

and types of feedstock used during production.11-13 Relatively modest concentrations of Ni and

71

Zn in water (> 10 µg/L for Ni and > 5mg/L for Zn) are shown to have toxic effects on human

72

health. Geogenic and anthropogenic inputs, especially from tanneries, smelters, or sewage sludge

73

application, can increase Ni and Zn concentrations in the environment.14-15 Therefore,

74

understanding the mechanisms by which environmentally relevant divalent metals such as Ni and

75

Zn are removed from aqueous solution by biochar is critical to assessing its use as an adsorbent.

76

Key to developing rigorous geochemical models that can accurately predict the removal

77

of metal cations from solution by biochar is the identification and quantification of surface

78

functional groups (sites), their protonation constants, and the reactivity towards specific

79

metals.16-27 Metal adsorption data can be modeled using two general categories of models: (i)

80

empirical models such as isotherms such as Freundlich and Langmuir, and (ii) thermodynamic

81

approaches such as surface complexation modeling (SCM). Empirical models normally cannot

82

account for changes in metal concentration, pH, ionic strength, temperature and complexation or

83

for the effect of competing ions for sorbent sites. For this reason, the application of empirical

84

partitioning constants to predict metal distribution in dynamic environmental systems can be

ACS Paragon Plus Environment

3

Environmental Science & Technology

85

problematic.16-17 In contrast, the SCM approach has been proven to be an effective method to

86

predict the acid/base and ion/metal binding behaviors of a variety of environmental surfaces, and

87

in systems approaching the complexity of those found in nature.16-26 The primary advantage of

88

using a SCM approach over empirical approaches is that the distribution of metals in a system

89

can be predicted using the stability constants determined experimentally for individual sorbent-

90

metal surface complexes, because the surface complexation theory is grounded in balanced

91

chemical reactions, and ultimately, in chemical thermodynamics.16-17

92

Recent studies which have modeled metal sorption by biochar have primarily relied on

93

empirical metal adsorption models.2-3,5-7,11,26 To our knowledge, few studies have applied a SCM

94

approach to biochar: Zhang and Luo28 modeled copper (Cu) adsorption to biochar; Vithanage et

95

al.29 modeled antimony (Sb) adsorption to biochar; and Alam et al.30 modeled the adsorption of

96

selenium (Se) and cadmium (Cd) to biochar-amended agricultural soils. While these studies

97

proposed SCMs that successfully describe the adsorption of metal ions onto sorbents across

98

variable chemical conditions, they do not provide direct measurements of the surface

99

coordination of adsorbed ions.

100

Here we apply two thermodynamic approaches, (i) surface complexation modeling

101

(SCM) and (ii) isothermal titration calorimetry (ITC), supported by synchrotron-based X-ray

102

absorption spectroscopy (XAS) to develop a predictive and mechanistic model of metal binding

103

to biochar. Synchrotron-based extended X-ray absorption fine structure (EXAFS) studies are a

104

powerful tool to identify the atomic metal-surface coordination environment.31-34 ITC

105

measurements were further conducted to determine the thermodynamic driving force (e.g., bond

106

formation, dehydration, enthalpy and entropy) of the metal-surface reaction(s). Understanding

107

the thermodynamic driving forces of adsorption provides critical insights into the reasons why

ACS Paragon Plus Environment

4

Page 4 of 34

Page 5 of 34

Environmental Science & Technology

108

spontaneous adsorption reactions occur, as well as the temperature dependence of these

109

reactions.35-39 In this study, we aim to: (1) determine whether the surface complexation modeling

110

approach can accurately predict Ni(II) and Zn(II) adsorption (as model divalent cations) to five

111

types of biochar, (2) explore the metal-biochar surface reactions at molecular level, and (3)

112

determine the thermodynamic driving forces of metal-biochar surface reactions. Using a varied

113

set of biochar allowed us to investigate the potential effects of production scale, differing sources

114

of biomass, and pyrolysis temperature on the metal adsorption behavior. Our study is the first to

115

develop a predictive model of metal adsorption to biochar underpinned by a mechanistic

116

understanding of the adsorption processes at the surface of biochar. As such, our work opens the

117

door to future studies that use the SCM approach as a flexible, predictive method to determine

118

the removal of metals from water by biochar in variable water chemistries.

119

2. MATERIALS AND METHODS

120

2.1 Biochar Preparation

121

Biochar produced from wheat straw (WS) and wood pin chips (WPC) were obtained from the

122

Alberta Biochar Initiative (ABI; Vegreville, Alberta, Canada). The raw feedstocks of WS and

123

WPC were placed in a prototype 1.0, batch carbonizer (Alberta Innovates Technology Futures,

124

AITF), and in an auger retort carbonizer (ABRI- Tech, 1 Tonne Retort system; ABI, Vegreville,

125

Alberta), respectively, and pyrolized under limited oxygen conditions. The residence time in

126

both cases was 30 min at 500°C to 550°C. The biochar yield was 30 to 33% on the basis of dry

127

mass. Sewage sludge biochar (SSBC) was produced at the Korea Biochar Research Center,

128

Kangwon National University, at a lab scale as described in Ahmad et al.12 The raw feedstocks

129

were ground to Si-OH) which

245

would be proton-active and could complex metals.4

246

3.2 Potentiometric Titrations

247

All biochar types studied showed significant buffering capacity from pH 4 to 10 (Figure S6).

248

Minimal variance was observed between forward and reverse titrations, suggesting reversibility

249

of the protonation and deprotonation reactions on the biochar surface during the timescale of the

250

experiments. Potentiometric titration data was modeled using a non-electrostatic, discrete site

251

surface complexation model to determine the proton binding constants and site concentrations

252

for the reactive surface sites for each biochar (Table 1). We defined three discrete site functional

253

groups from lowest to highest pKa (-log Ka) values for all five types of biochar. The three-site

254

models were calculated using a least-squares optimization routine, as implemented in FITEQL

255

4.0.49 In all cases the variance, or V(Y), values were in the range of 0.1